I've successfully baselined the idle by cleaning the throttle body and by adjusting the throttle stop screw, throttle actuator rod and brand new TPS to Volvo's specifications.

The IAC has been thoroughly cleaned overnight and reinstalled. Once the carburetor and injector cleaner ran clear, I stopped. The vacuum hoses have been checked for cracks and clamps re-recorded. For good measure, the bolts that secure the throttle body gasket have been checked.

Although it may not be directly relevant, the air filter, spark plugs, wires, distributor cap and rotor, fuel pump and filter have been replaced. The ground straps have been checked to ensure they are secure. Electric connectors going to vital components and sensors have been cleaned with the appropriate chemical sprays.

Regardless, on cold start up, it vibrates but holds a correct idle of ~750 RPMs in park, neutral, drive or reverse. Not long after, and once warmed up, idle maintains itself at ~950-975 RPMs in drive or reverse, and ~1250 RPMs in park. If I apply a clamp to the IAC hose, the idle drops to ~500 RPMs, which seems correct.

At this point, I'm not sure what to do. I'm hoping for a clue to help me figure this out. If not, I either have to take it to my mechanic or simply sell it. At this point, I'm out of patience and willingness to invest more time with no results. Thank you for taking the time to read this.

Not for the car's sake, but my own sanity, I tried one more time to adjust the TPS and tried a little experiment. Instead of a .25mm feeler gauge, I tried a .33mm feeler gauge between the throttle lever and idle stop screw. The idle improved somewhat, sticking to somewhere between 1000-1100 RPM in park while arm. Something is obviously out of adjustment for this to help, but it's certainly interesting. Even more interesting because I brought it down to base idle to make sure there wasn't anything wrong otherwise.

Since I got such great help in such a short time frame, I figured I'd ask something else that's been gnawing at me. It's about the TPS adjustment guide for the LH2.4 system you find on forums and FAQs all over the internet:

"4. Adjusting the TPS.

Loosen the TPS adjustment screws. First turn the TPS clockwise (away from the electrical connector) until it stops. While keeping your finger on the throttle disk so it won't move, turn the TPS counter-clockwise (toward the connector) until you hear or feel a "click". Continue turning until it stops, then tighten the screws."

Everything makes sense except the last sentence, whether "it" is the click or the TPS. Are you supposed to continue turning the TPS until the click stops or the unit can no longer be rotated counter-clockwise? That's two very different things. I assume it's referring to the click, but it either clicked or it didn't (not a duration thing). Otherwise, continuing to rotate the TPS past the click would defeat the purpose of finding the click in the first place.

Sometimes, I have too much time to analyze things, or I overthink stuff, but I was just wondering. I had already adjusted the TPS to where it passes the feeler gauge test and many folks say not to set it "hair trigger precise" but I was messing with it the other day and want to re-set it correctly without having to mess with feeler gauges again.

"Not long after, and once warmed up, idle maintains itself at ~950-975 RPMs in drive or reverse, and ~1250 RPMs in park. If I apply a clamp to the IAC hose, the idle drops to ~500 RPMs, which seems correct."

High idle (~1250 rpm) in park means the ICV not working. You'll get same result if you disconnect the ICV electrical plug and start the engine. But in your case after replacing with a known good part with still no changes means the TPS is not adjusted properly. Plus having this issue only when engine warmed up means the TPS is not closed during this time.


"Continue turning until IT stops, then tighten the screws."

IT refers to the TPS.


While making TPS adjustment be sure to:
1) put 0.25mm feeler gauge between the throttle lever and stop screw.
2) maintain some finger pressure on the "throttle lever-feeler gauge-stop screw" thing because if you keep turning the TPS counter-clockwise the throttle plate will start to swing open a bit thus disturbs the final adjustment.

I think the 0.25mm feeler gauge is for temp compensation i.e when engine is hot the stop screw will lengthen a bit. Thus if you set the TPS "hair trigger precise" or without the feeler gauge in place, the lengthened stop screw would never let the TPS close at throttle closing, leading to high idle when engine is hot. This may also give the impression that the ICV is faulty whereas it is not. Its just not activated because TPS never closed.

The important starting point is the stop screw (sorry to bring this up again - just to be sure). Easier to adjust when throttle dismantled out of engine. Throttle insides must be throughly cleaned or else the plate can't be fully closed. You can feel some resistance to turn as soon as the stop screw touches the throttle lever. Easier to confirm this visually with throttle out of engine. From here turn the screw a quarter more so that the throttle plate's tension is resting on the screw and not on the insides of throttle. Lock the screw. Leave this setting no matter what idle issue you're having. Next adjust the TPS as above.

Hope this helps,
Amarin.

Hi amarin, thanks for the thoughtful and insightful response. I pulled the TB and followed your instructions to the letter, ensuring it was clean and the gasket to intake manifold good and sealed. Unfortunately, the problem persists after retesting and a fruitless search for vacuum leaks. I appreciate everyone's input, but this was only a $400 car and simply not worth the trouble it's giving me. It will be listed for sale tomorrow.

Dear mphilleo,

Hope you're well. The Idle Air Control Valve has failed (IACV). Installation of a spare, "known good" IACV, should produce a steady idle at 600-700 RPM. The clue: cleaning the IACV made no difference and clamping the hose to the IACV corrects the problem.

Hope this helps.

Yours faithfully,

Spook

The brand new Bosch IACV and several attempts for verification of correct TPS and throttle stop screw adjustment has yielded the same result. I will likely return the part and sell the 940 at this point. Thank you for your time and assistance.

mphilleo,

Please let the brickboard folks help you through your idle control and other issues you may have with your 1991 Volvo 940 before considering a sale.

Properly diagnosed repairs may be more economical than purchasing another used automobile that may require repairs you may not be able to diagnose before the time of sale.

Other issues can affect idle control on your 1991 Volvo 940.

https://www.brickboard.com/FAQ/700-900/

You may want to check ground connections on the 1991 Volvo 940. Usually black wires from the sensor or device requiring a ground external to the wire harness, such as engine control, that leads to a self-tapping sheet metal screw or machine screw in an embedded fastener.

http://www.volvowiringdiagrams.com/?dir=volvo/940%20Wiring%20Diagrams

Wiring diagrams may not mention where the ground connections are, well, connected.

Good that you cleaned connectors and such. The bare braided tinned over copper flexible braided ground straps between the engine and body, or elsewhere, do provide ground, yet between the engine and body, they reduce RF interference to your radio.

Have you resolved vacuum leaks? Checked the OBD for fault codes?

Continue with the throttle position switch (TPS). These can fail before the OBD catches up. Same with other intermittent failures the OBD actually monitors.

You can clean an IAC using throttle body cleaner. These IACs can foul, restricting motor movement range, if you use mineral, and not synthetic motor oil, or a prior owner did not change the mineral motor oil regularly. Though you've found with a new IAC you suffer the same problems.

At any rate, your problem may most likely be electrical within the engine control wire harness, as you've discovered.

I dunno whether you have:
- Turbo with LH-Jetronic fuel control 2.4 / EZK 116 ignition
- LH-Jetronic fuel control 2.4 / EZK 116 ignition (with or without or EGR)
- Regina fuel control / Rex ignition

Please check your OBD for fault codes. Report back.



An issue with a 25-year old Volvo 940 (and all 700-series), is corrosion at the fuse - relay / platform thingy behind the ashtray at the bottom of the center dash console (or on the right side for the fancier models and newer). Where the relays sit on the platform, the pins, where they make electrical connection with the receiving spade terminals of the wire harness, embedded in the blue plastic ABS platform, can form corrosion, reducing current flow. You may want to remove and replace the relay, very gently, to renew the bonded electrical interface. Also, on Bosch and I believe Regina, the IAC is powered through the fuel injection relay. OBD cannot report such a fault. Though you can check and repair the fuel injection relay if you know how to solder. In some instances, the IAC may be grounded through a relay or resistor. (See wiring diagram, so aforementioned.)

If you see heat warping and such around a rely, you may have found a problem. See dielectric DeoxIT to cure and and prevent connection corrosion problems on low voltage DC. (You may have already used.)

Relay Platform diagram (Unless you have 940 SE):


See Dave Barton's web page on Volvo relays:

http://www.davebarton.com/volvorelays.html

Dave has one image for 940 and a shared version for 700/900.

http://www.davebarton.com/pdf/700-900relaylocations.pdf

Also, these relays, on the platform behind the ashtray, share some common grounds that are collected near the platform and secured on the body someplace. Grounds can also form corrosion and the securing screw or bolt can loosen. A merely loosen and tighten, gently, can restores a good bonded electrical contact.



Sorry to go on so long.

Questions?

Hope that helps.

Flax Seed Boy.
--
Jonathan Harshman Winters III: The Mightiest, Greatest, & Most Powerful North American Comedian & Comedic Actor in Perpetuity

Hi Spook, thank you, I am well and hope you are too. I appreciate the quick response and insight. To be honest, I had so much "if not X, then maybe Y?" going on in my mind, I didn't even put 2 and 2 together. It would make sense, the way you explained it, that what I described points to a faulty idle air control valve. I'm so confident that that is probably the issue that I ordered a replacement Bosch part this morning. It should arrive tomorrow and I look forward to resolving my idle problem (or at least eliminating another factor). Thanks again!